1. Field of the Invention
The present invention is directed to underground heat exchange systems; to apparatus and methods for installing a loop of pipe in a hole in the earth; and to thermally conductive filler materials for emplacement between the pipe loop and the interior hole wall.
2. Description of Related Art
The prior art discloses a wide variety of earth heat exchange systems and methods for using energy transferred by such systems.
The prior art discloses the use of a common grout, typically a bentonite clay mixture, for use as a thermally conductive material between a pipe loop of an underground heat exchange system and the interior of a hole in which the loop is positioned.
The cost for the installation of certain prior art vertical ground source heat loops can account for nearly half of the total cost incurred in the installation of a geothermal heat pump system. In certain aspects, the heat loop installation consists of drilling a vertical well, placing a thermoplastic pipe "loop" in the wellbore, and filling the annular space between the loop and the well wall with a thermally conductive material. Several problems can arise in such methods: loop "insertion" difficulties caused by loop buoyancy; thermal conductivity and installation difficulties of borehole backfill materials; and environmental concerns.
Although the drilling of the hole may be relatively easy, certain problems may be encountered in inserting the loop. In most application, the "loop" consists of a relatively lightweight thermoplastic pipe. One material of choice is polyethylene, with a specific gravity of approximately 0.955 gms/cc.sup.3 lighter than water. This pipe material also has relatively poor stiffness. As a driller attempts to push the pipe down into a mud filled well, the natural buoyancy of the pipe resists these efforts. Since the pipe has poor longitudinal stiffness, the pipe tends to bend or curl inside the well, creating additional frictional drag against the well wall until the pipe can be pushed no deeper. Even when the pipe is filled with water, the loop still maintains considerable buoyancy because the drilled hole is filled with dense drilling mud. In soft geological formations, the driller must mix heavy drilling mud in order to transport drilled cuttings and to stabilize the hole. In most cases, bentonite clay is added to the drill fluid to reduce friction, prevent loss of circulation, and suspend solids. However, the result is residual heavy drilling fluid in the well, creating a very dense slurry of clay, sand, rock, etc.--with resulting high buoyancy and low surface tension. In order to overcome the buoyancy problems, the driller usually attaches a heavy steel bar to the leading end of the heat loop and "pulls" the loop to the bottom of the well. Once the loop hits bottom, the driller 1) remotely secure the loop in the hole to prevent it from floating up, 2) remotely detaches the steel bar from the loop, 3) and recovers the bar from the well, usually by a cable winch. The considerable efforts made to overcome buoyancy of the loop caused by the density of the drill fluid are time consuming, expensive, and hazardous to the integrity of the loop.
The ability of the loop to transfer heat is directly related to the efficient operation of a geothermal heat pump system. Great expense and many tests and studies have been performed in a search for an optimum thermally conductive material that can be economically placed in the annular space between the loop pipe and the wellbore wall. An ideal material would be: at least as thermally conductive as the native earth; be easily and reliably placed in the borehole; maintain conductivity "long term", and be inexpensive.
In some applications, particularly those where a loop is installed below a water table, no "bore backfill" material is placed in the well. The water is extremely thermally conductive (if it remains in the bore long term). The concern is that over time, the water table may drop, leaving an insulating airspace between the loop and the borehole. This insulating airspace results in "hot loops", and the heat pump system either works poorly or not at all.
Many conductive high solids and cementitious "grouts" have been developed which include a mixture of water and a relatively conductive solid such as silica sand or fly ash. In order to transport the heavy solids, a viscosity enhancer such a bentonite clay is added. Also, friction reducers and "super-plasticizers", like sulfonated naphthalene, are added to make the slurry pumpable. Although these grouts seem to work well in testing, there is great resistance from the field installers. The mixture can be expensive to handle, difficult to mix, even more difficult to pump, and extremely rough on equipment because of the abrasion. In addition, placing of the material in the borehole is difficult to control and monitor. If grouting is attempted from the surface, "bridging" can occur resulting in a partially filled hole. If grouting from the bottom of the hole, "channeling" can occur, again resulting in a partially grouted hole. When the water subsides in the borehole, this leaves air spaces, thus insulating the loop and reducing the efficiency of the system. An additional disadvantage is that the voids from channeling as well as the interstitial spacing between the individual sand (or other solid particles) create permeability. Permeability that creates a vertical communication path by which the ground water system could be contaminated by surface spills is an environmental concern.
The prior art cementitious grouts, developed to overcome the permeability objection, have some unique problems of their own. In addition to all of the handling, mixing, pumping, abrasion, and conductivity problems of the High Solids Thermally Conductive Grouts, the "heat of hydration" generated when cement cures, causes grout to "shrink" away from the polyethylene pipe. As heat is generated, the polyethylene pipe, having a very high coefficient of expansion expands. Once the cement cures and cools, the polyethylene contracts and actually pulls away from the grout. Although the permeability of the cured grout itself is very low, a flow path may now exist between the polyethylene pipe and the grout--again insulating the loop and threatening the environment. Because of its inherent structure, polyethylene is a very high molecular weight wax or paraffin, and does not bond well with anything, even under laboratory controlled conditions. Attempts to control grout shrinkage and cement to polyethylene bonding in the field were proven unsuccessful.
Drilling "polymers" have existed for years. They have been used in the oil well drilling industry as an alternative to bentonite clays to provide solids transport, solids suspension, friction reduction, and displacement efficiency.
The prior art discloses a variety of systems and apparatuses for installing ground heat exchange pipe loops in a wellbore, including a system in which a wellbore is drilled, e.g. a vertical hole four to four-and-a half inches in diameter to a depth of about 250 feet, and a single piece of polyethylene pipe attached to a sinker bar is introduced into the hole and then pulled out of the hole manually while grout is introduced into the hole. A pipe loop (polyethylene) is pushed to the bottom of the hole by a wire-line retrievable sinker bar. With the sinker bar removed, a series of screwed together 2 inch PVC tremmie pipes is lowered to the bottom of the hole and grout mixed at the surface is pumped into the tremmie pipe. As each batch is pumped into the hole the tremmie pipe string is raised and one 20 foot section of pipe is removed from the hole. After grouting is completed and the tremmie pipe is removed, the rig is moved to another drilling position, e.g. at least 15 feet away. When all of the pipe loops have been installed (e.g. one loop for each ton of heating and cooling equipment), the drill rig is removed from the site. A trench (e.g. about four feet deep) is then dug to contain pipes that interconnect all of the pipe loops and a connecting pipe is laid into the trench, heat fused to each of the vertical pipe loops, and pressure tested and buried to serve as a circulating manifold carrying water between the earth and a heat pump located within an adjacent building. The trenching and manifolding of the surface pipe typically takes as much time as the wellbore drilling and pipe installation.
The prior art discloses numerous in-ground heat exchanger systems (e.g. see U.S. Pat. Nos. 5,244,037; 5,261,251); and grouting systems (see, e.g. U.S. Pat. No. 5,435,387).